1. Field of the Invention
This invention is in the field of coating of substrates like pharmaceutical tablets, and it is specifically concerned with coating such substrates with a coating that acts as a moisture barrier.
2. Description of the Prior Art
Many medicinal substances and the dosage forms into which they are made undergo degradation during storage due to contact with moisture which has been absorbed from the atmosphere. To combat this degradation, it is frequently necessary for such moisture sensitive dosage forms to be packed in special containers to limit the exposure of the dosage forms to the atmosphere.
In territories where the climate is very humid, specialized packaging does not provide a totally satisfactory answer to moisture degradation.
Elimination of the need for the special packaging, or augmentation of the moisture protection provided by such packaging, may be achieved in the case of solid dosage forms, such as tablets, capsules and granules, by coating with materials which reduce the rate at which the dosage form absorbs atmospheric moisture. Moisture barrier coatings of this type have been employed previously. The traditional means of providing the moisture barrier characteristics required for moisture barrier coatings has been by the use of polymers which are soluble in organic solvents or are relatively insoluble in water. The disadvantages of this approach are 1) the need to use either organic solvents or difficult aqueous polymer dispersion techniques to apply these water-insoluble polymer materials, and 2) the reduced aqueous solubility of the coating leading to prolonged disintegration time of the dosage form in the body when ingested, with consequent delayed or reduced activity of the medicinal substance in the tablet.
The use of the polymer polyvinyl alcohol, PVA, as a moisture barrier coating has been previously suggested. However, practical usage has been inhibited by the stickiness of grades of the polymer which have a fast enough rate of going into solution in water to make a dispersion to render them economical to use in making the coating. A further problem with the use of PVA is in identifying or selecting a plasticizer which does not compromise the moisture barrier properties of the final coating.
A dry powder moisture barrier coating composition is made to form a moisture barrier film coating for pharmaceutical tablets and the like, which comprises polyvinyl alcohol in combination with soya lecithin, together with, none, one or more of the following components which enhances the desirable coating characteristics of the resulting film coating: a flow aid, a colorant, and a suspending agent.
A method of making the moisture barrier film coating powder composition of the invention comprises the steps of mixing polyvinyl alcohol with soya lecithin and optionally with one or more of the following components, until a dry homogenous powder mixture is produced: a flow aid, a colorant, and a suspending agent. Preferably, the mixture of polyvinyl alcohol and soya lecithin, or the mixture of polyvinyl alcohol and soya lecithin and any of the following components, if any of the following components is to be included in the coating composition, is milled: a flow aid, a colorant, and a suspending agent. The resulting dry powder coating composition is readily soluble or dispersible in cold water to form a liquid coating solution or dispersion that is ready to use within 45 minutes.
Another method of making a dry edible film coating composition of the invention comprises producing a mixture of the ingredients processed into a granular form to form a granular coating composition by using any of the following methods: wet massing, fluid bed granulation, spray granulation and dry compaction, roller compaction or slugging.
The invention also includes a liquid coating solution or dispersion for forming a moisture barrier film coating which comprises polyvinyl alcohol, soya lecithin, an optional flow aid, an optional colorant, and an optional suspending agent mixed into water.
The method of making the liquid coating dispersion of the invention comprises dispersing the dry powder coating composition, the granular coating composition, or the individual ingredients of the coating composition separately, into hot or cold water, and/or milling and/or stirring until a homogenous mixture of a desired correct viscosity is produced.
The film former of the coating is the polyvinyl alcohol, which may be GOHSENOL polyvinyl alcohol made by NIPPON GOHSEI, for example.
Excellent moisture barrier properties are obtained when hot water soluble grades of PVA are used in the inventive coating, but use of such grades requires that the water of the liquid coating dispersion be heated. The coatings of the invention may use cold water soluble grades of PVA to provide moisture barrier protection, but they provide less moisture barrier protection than that provided by the inventive coatings that use hot water soluble grades of PVA. A preferred grade of PVA for use in the inventive coating is a grade in the medium range (i.e., the grades of PVA between the grades that dissolve only in hot water and the grades that dissolve very easily in cold water) of PVA because the step of heating the water of the liquid coating dispersion may not be necessary, while still maintaining excellent moisture barrier properties in the inventive coating.
The flow aid may be talc, fumed silica, lactose or starch.
The colorant may be any food approved colors, opacifiers, or dyes. For example, these may be aluminum lakes, iron oxides, titanium dioxide, or natural colors.
The soya lecithin, which may be ALCOLEC soya lecithin made by American Lecithin Company, for example, acts an a anti-tack agent, and surprisingly, and unexpectantly, acts as a plasticizer by locking moisture in the coating so the coating stays flexible and not brittle. Surprisingly and unexpectantly, this locked-in moisture in the coating, while contributing to plasticization, does not compromise the moisture barrier properties of the overall coating.
The suspending agent acts as a viscosity modifier stabilizing the coating dispersion. A preferred suspending agent is xanthan gum. Examples of alternative suitable suspending agents include hydroxypropylmethlycellulose (HPMC), alginates, hydroxypropyl cellulose (HPC), natural gums, and carboxymethylcellulose.
The ranges for each component of the dry coating compositions of the invention are as follows, by weight:
The ranges for each component of the liquid coating dispersions of the invention are as follows, by weight: